There are a number of applications in which it is desirable to provide a constant voltage to a load. It is preferable to connect two power supplies to a common load to increase the lifetime of each supply and provide a backup if one fails. However, coordination of the power outputs of dual power supplies presents several problems.
Most power supplies are provided with positive and negative sense terminals which connect directly to the load. By connecting directly to the load, the sense terminals monitor the voltage across the load itself and eliminate effects of impedance in the output lines between the power terminals and the load. The power supply holds its output voltage steady when the voltage across the positive and negative sense terminals equals the rated voltage of the power supply. A difference between the sensed voltage and the rated voltage causes the power supply to alter its output.
When the sense terminals of two power supplies are connected independently to a common load, discrepancies in the output currents can arise: frequently, misadjustments in one or both power supplies cause the power supplies to seek a sensed voltage which is different from the expected, rated voltage. More strain is then placed on one of the power supplies if the other power supply provides less than the rated voltage, and improper voltages may be applied across the load.
Several conventional systems attempt to balance the current outputs to and voltage across a common load. One utilizes a master-slave arrangement in which the master power supply commands the slave to track a designated amount of current. However, if the master power supply fails or is switched off, the slave power supply simply tracks the output of the master to zero current rather than compensating for the diminished current. This arrangement is used primarily to provide a higher level of current than is available from a single power supply.
Another system utilizes load sharing circuits which are located within each of two power supplies and communicate directly with each other. Each load sharing circuit works with the other circuit to control the current output of both power supplies. Since these load sharing circuits are integrated into their respective power supplies, they are dedicated to the rated voltages and currents of those power supplies. Consequently, the load sharing circuits are not transferable and, even if they were, they could not accommodate different voltage and current environments.